Natural gas, a combustible mixture of hydrocarbons, is a very important source of energy since it is clean, cheap and efficient. The major component is methane, but it may also contain small amounts of other hydrocarbon compounds such as ethane or butane. A natural gas is described as sweet (with low sulfur contents) or sour (with high sulfur contents). It may also be wet or dry, depending on the presence of natural gas liquids and other energy gases. When more than 90% of a natural gas is composed of methane, it is referred to as dry.

There are three theories that explain the formation of natural gas. The first is that natural gas is formed when organic matter, such as the remains of a plant or animal, is compressed beneath the earth at high pressures for a long period of time. This is referred to as thermogenic methane.

Another theory suggests that natural gas is formed by the decomposition of organic matters by a microorganism. These microorganisms chemically break down the organic matters into pure methane, which is referred to as biogenic methane.

The third states that methane is formed by the reaction of hydrogen rich gases and carbon molecules deep inside the earth. In the absence of oxygen, they may combine to form hydrocarbon gases. Under high pressure, these gases may rise to the surface of the earth and form methane deposits.

Energy density is measured by the amount of energy stored in a given unit of matter or system. For natural gases, the energy density is the either the amount of energy stored per unit volume or per unit mass of the gas. The energy stored per unit volume is usually measured in British Thermal Units per cubic feet, or, the amount of natural gas that will produce enough energy to heat one pound of water one degree at normal pressure. The standard unit is megajoules per cubic meter. The energy density of a natural gas lies in the range of 900-2200 Btu/ft3or 33.4–82.7 MJ/m3.